In order to examine a body of a subject of interest, typically a patient, using magnetic resonance imaging (MRI), different magnetic fields, which are tuned to one another as precisely as possible with respect to their temporal and spatial characteristics, are radiated onto the subject of interest. A powerful main magnet generates a powerful static main magnetic field B0, which typically is provided with a magnetic field strength of 1.5 Tesla or 3 Tesla, in some embodiments with a magnetic field strength of more than 3 Tesla. The subject of interest is typically placed on a vertical support and moved into an approximately homogeneous region of the main magnetic field B0 in a field of view within a bore of the main magnet. The nuclear spins of atomic nuclei of the subject of interest are excited by magnetic radio-frequency excitation pulses B1 (x, y, z, t), which are radiated into the nuclei via a radio-frequency antenna and/or a local coil arrangement. Still further, high frequency excitation pulses are generated and guided to the radio-frequency antenna.
The MRI system further includes gradient coils, with which magnetic gradient fields BG(x, y, z, t) are radiated during a measurement for selective slice excitation and for spatial encoding of the measuring signal. Signals emitted by the excited nuclear spins of the atomic nuclei in the subject of interest to be examined are received by at least RF receive coil, amplified, further processed and digitized. The recorded measured data is digitized and stored as complex numerical values in a k-space matrix. An associated MR image may be reconstructed from the k-space matrix containing the complex numerical values by, for example, a multidimensional Fourier transformation.
Local coils, which are coils not permanently connected to the MR imaging system, are employed according to the use of the MR imaging system. Accordingly, the MR imaging system comprises a socket and the RF receive coil comprises a plug for creating a connection between the local coil and the MR imaging system. When a local coil is introduced in the bore of the MR imaging system, and not connected to the MR imaging system via the plug and the socket, the subject of interest may be endangered or the local coil maybe damaged or even destroyed.
This occurs since MR receive coils are often in a “tuned” state by default. Tuning refers to an adjustment to a desired RF coil frequency used in the MR imaging system for exciting the nuclei of the subject of interest and for receiving MR signals from the nuclei of the subject of interest. As a consequence, very high field strengths during MR-transmit phases of other coils, e.g. the QBC, can be created locally. This is a safety concern, since the applied RF fields can generate high currents in the RF receive coil, which can generate heat, which represents a risk for the subject of interest. Furthermore, these currents are problem for the RF receive coil, since the currents may damage or even destroy the RF receive coil. Furthermore, the presence of not connected RF receive coils in the bore of the MR imaging system may reduce the performance of the MR imaging system, since the currents introduced in the unconnected RF receive coil may result in error signals picked up by other, connected RF receive coils. In that case, if the error is not detected prior to terminating the MR imaging sequence, the MR image data may be entirely lost.
The most relevant measure that prevents the problem, is that the local coil is actively detuned during phases not used for receiving signals, e.g. during transmit phases of other coils. This active detuning requires a signal and power control from the MR imaging system, which provided via the socket and the plug of the RF receiving coil connector and a connection lead to the local coil, i.e. the RF receive coil. Hence, when the RF receive-coil is inserted in the MR imaging system while unconnected, this safety measure cannot be working.
That concern can be mitigated by passive detuning, which will reduce the coil-response during transmit of other coils. This is typically achieved by steering capacitor diodes of the RF receive coil using high DC currents, so that high currents through the RF receive coil can be avoided or at least reduced. Nevertheless, this requires a modified design of the RF receive coil and does not solve the problem of reduced performance of the MR imaging system based on the unconnected RF receive coils in the bore.
In the case that this protection by active and passive detuning fails, fuses may, be installed in the RF receive coil that blow if the HF currents in this local coil are too high to protect the subject of interest and to protect the RF receive coil itself. Nevertheless, when the fuse blows, The RF receive coil is not usable and maintenance by a technician is required to make the RF receive coil usable again.